A known seat belt system for restraining an occupant of a vehicle includes seat belt webbing, a seat belt buckle, and a webbing retractor. A tongue is connected to the webbing and is releasably lockable in the buckle when the webbing is extended across the vehicle occupant. The retractor includes a spool upon which the webbing is wound. The spool rotates in a webbing withdrawal direction when the vehicle occupant extracts the webbing from the retractor. A rewind spring in the retractor rotates the spool in a webbing retraction direction to retract the webbing into the retractor.
When the vehicle experiences sudden deceleration, a vehicle occupant using the seat belt system applies a force against the webbing. The force which is applied to the webbing urges the spool to rotate in the withdrawal direction. The retractor includes a blocking mechanism which blocks rotation of the spool in the withdrawal direction in response to sudden vehicle deceleration. Thus, the blocking mechanism prevents further withdrawal of the webbing from the retractor, and the webbing restrains forward movement of the vehicle occupant.